Many permutations and combinations exist in astrology. I am just giving one example as to how your date of birth  affects your nature.

Sun sign astrology is the form of astrology most commonly found in many newspaper and magazine columns. It is a simplified system of astrology which considers only the position of the Sun at birth, which is said to be placed within one of the twelve zodiac signs.

If you are born from March 21 to April 20.Then it translates into Sun in Aries sign 

Sun in Aries is exalted  that tends to give strong will power and desire to rise up in life.

Aries The Ram   ( March 21 to April 20 ) 

Aries Traits

Adventurous and energetic

Pioneering and courageous

Enthusiastic and confident

Dynamic and quick-witted

Aries On the dark side...

Selfish and quick-tempered

Impulsive and impatient

Foolhardy and daredevil 

Many things affect your nature. This is just one component of the horoscope.